This invention relates to treating of exhaust gases containing fluorine-containing compounds. More particularly, the invention relates to a method and apparatus for treating exhaust gases containing fluorine-containing compounds that are discharged in the semiconductor industry from the step of dry cleaning the inner surfaces and the like of semiconductor fabrication equipment and from the step of etching a variety of films including oxide films.
In the semiconductor industry, a great variety of harmful gases are used during the semiconductor fabrication process and those gases, if simply discharged into the environment, may potentially pollute it. Particularly in the step of cleaning the inner surfaces of semiconductor fabrication equipment and in the etching or CVD step, fluorine-containing compounds including hydrofluorocarbons such as CHF3 and perfluorocarbons (PFCs) such as CF4, C2F6, C3F8, C4F6, C4F8, C5F8, SF6 and NF3 are employed; the fluorine-containing compounds contained in exhaust gases from those steps are global warming gases and there is an urgent need for developing an effective system to remove them.
With a view to removing PFCs from PFC containing gases, the present inventors previously invented a method for thermally cracking PFC in the presence of both O2 and H2O using a γ-alumina catalyst having a specified crystalline structure and applied for a patent on the method (Japanese Patent Application No. 2000-110668). A method has also been proposed for treating non-PFC flon gases by cracking flons with a cracking catalyst carrying a tungsten oxide on an alumina-zirconium composite oxide in the presence of a hydrocarbon such as n-butane and O2 being used as cracking assist gases.
When the γ-alumina catalyst having the specified crystalline structure was used in PFC treatment according to the proposal made by the present inventors, the γ-alumina showed high enough cracking activity to achieve complete (100%) PFC removal, with no CO generated as a by-product. In spite of this high efficiency in treatment, the present inventors found by later studies that the γ-alumina gradually deteriorated as the passage of gas was prolonged and that the percentage of gas removal dropped within a shorter time than it was initially desired.
As for the metal catalyst carrying a tungsten oxide on an alumina-zirconium composite oxide, its use in the treatment of flon gases (e.g. flon-115) has been proposed but no review has been made about its applicability to refractory PFCs (CF4, for example, must be cracked by heating to at least 1200° C.). In the proposed method, a hydrocarbon and O2 are used as cracking assist gases but no review has been made about using H2O in place of the hydrocarbon. Hence, the metal catalyst carrying a tungsten oxide on an alumina-zirconium composite oxide has not been studied at all from the viewpoint of its activity in cracking PFCs in the presence of H2O and O2.